In many operating procedures, it is often necessary to introduce catheters into tile large veins of the body and into the arterial vessel system. Arterial and venous catheters are particularly useful for cardiac catheterization and other radiologic procedures such as cerebral angiograms. It is well established that the insertion of arterial and venous catheters for various purposes such as for angiography can be responsible for patient discomfort because locating and penetrating arteries and veins can be especially difficult when dealing with patients who are obese or present an unusual anatomical situation.
Repeated attempts at penetration may be required for vessels which are difficult to reach. Multiple blind penetrations can cause injuries even extending to an occlusion of the vessel concerned. Further, frustrated attempts at penetration lead to loss of time, which is undesirable, for example, in the case of an emergency or even when preparing for an elective operation.
To avoid difficulties when localizing the vessel and to reduce the risk of complications, it is known to determine the position and the course of the vessel to be penetrated by means of ultrasonic Doppler sonography. A known apparatus emits an ultrasonic beam of a specific frequency from the skin surface toward the interior of the body. If any blood vessels are present within this ultrasonic beam, the emitted wave undergoes a Doppler effect due to flow through the blood vessels or the pulsation of the vascular walls, so that a reflected wave that has a different frequency from that of the emitted wave can be obtained.
After this reflected wave has been converted into an electrical signal with an ultrasonic oscillator, synthetic detection of the emitted signal allows an electric signal of the difference of the two waves to be obtained. This can be amplified and sent to a speaker or the like to produce a sound having a unique tone that can be detected by the ear. These sounds reach their maximum volume when the ultrasonic beam is directed toward the center of the artery or vein in question and cease if the ultrasonic beam strays from the vessel. Further, the reflected wave from tissue that has no movement cannot be heard nor can the sound resulting from vessels that are out of the line of the beam. Hence, Doppler sonography provides a simple means of localizing vessels both easily and accurately.
Certain applications of Doppler ultrasound utilize the transmission of ultrasonic waves through the needle and reception of ultrasonic echoes by a separate transducer located on the body of the patient separated from the syringe and needle. For example, U.S. Pat. No. 3,556,079 directed to a "Method of Puncturing A Medical Instrument Under Guidance of Ultrasound" discloses an apparatus wherein an ultrasonic beam is transmitted through a needle and the backscattered waves which have changed their frequency in accordance with the Doppler effect are received by an apparatus located on the body of the patient away from the needle. This patent also discloses the placement of both the transmitting and receiving transducers in the needle and syringe. However, this requires a special catheter construction which may give an erroneous signal when the needle engages the blood vessel before penetrating the vessel.
A major advance to Doppler technology was made by virtue of U.S. Pat. No. 4,887,606 directed to an "Apparatus for Use In Cannulation of Blood Vessels", which teaches the use of a transducer insert positioned within a hollow needle including an ultrasonic transducer at one end for transmitting and receiving ultrasonic waves through the sharpened end of the needle. Upon location and penetration of a blood vessel, the transducer insert is removable from the needle for implementation of the known Seldinger technique for placing a catheter in a blood vessel. Although the device disclosed in U.S. Pat. No. 4,887,606, the disclosure of which is incorporated by reference herein, represents a superior apparatus for cannulation of blood vessels, such apparatus could be improved upon. For example, the power emitted by a transmitting transducer is at least in part a function of surface area. Thus, larger surface area transducers can deliver more power and thereby increase the depth of penetration of the transmitted and reflected waves.
Since the reflected waves from small vessels that are located at large depths from the surface of the body are weak, it is important in certain applications to increase the depth of penetration of the transmitted and reflected waves. Moreover, with the known apparatus that utilizes a constant frequency transmission, it may be necessary to provide for somewhat sophisticated electronic amplification to sharpen the sensitivity of the receiving apparatus for the purpose of obtaining a strong detected sound. This obviously results in a more expensive and technically complicated receiving system. Therefore, there has been a need for an improved apparatus for the cannulation of blood vessels which is relatively easy to manufacture and has higher sensitivity than prior devices.